This invention concerns apparatuses and a method for removing solvent residues in particular after xe2x80x9cextractionxe2x80x9d of biomass. Biomass extraction is the extraction of flavours, fragrances or pharmaceutically active ingredients from materials of natural origin (these materials being referred to as xe2x80x9cbiomassxe2x80x9d).
Examples of biomass materials include but are not limited to flavoursome or aromatic substances such as coriander, cloves, star anise, coffee, orange juice, fennel seeds, cumin, ginger and other kinds of bark, leaves, flowers, fruit, roots, rhizomes and seeds. Biomass may also be extracted in the form of biologically active substances such as pesticides and pharmaceutically active substances or precursors thereto, obtainable e.g. from plant material, a cell culture or a fermentation broth.
There is growing technical and commercial interest in using near-critical solvents in such extraction processes. Examples of such solvents include liquefied carbon dioxide or, of particular interest, a family of solvents based on organic hydrofluorocarbon (xe2x80x9cHFCxe2x80x9d) species.
By the term xe2x80x9chydrofluorocarbonxe2x80x9d we are referring to materials which contain carbon, hydrogen and fluorine atoms only and which are thus chlorine-free.
Preferred hydrofluorocarbons are the hydrofluoroalkanes and particularly the C1-4 hydrofluoroalkanes. Suitable examples of C1-4 hydrofluoroknes which may be used as solvents include, inter alia, trifluoromethane (R-23), fluoromethane (R-41), difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1-trifluoroethane (R-143a), 1,1,2,2-tetafluoroethane (R-134), 1,1,1,2-tetrafluoroethane (R-134a), 1,1-fluoroethane (R-152a), heptafluoropropanes and particularly 1,1,1,2,3,3,3-heptafluoropropane (R-227ca), 1,1,1,2,3,3-hexafluoropropane (R-236ea), 1,1,1,2,2,3-hexafluoropropane (R-236cb), 1,1,1,3,3,3-hexafluoropropane (R-236fa), 1,1,1,3,3-pentafluoropropane (R-245fa), 1,1,2,2,3-pentafluoropropane (R-245ca), 1,1,1,2,3-pentafluoropropane (R-245eb), 1,1,2,3,3-pentafluoropropane (R-245ea) and 1,1,1,3,3-pentafluorobutane (R-365mfc). Mixtures of two or more hydrofluorocarbons may be used if desired.
R-134a, R-227ea, R-32, R-125, R-245ca and R-245fa are preferred.
An especially preferred hydrofluorocarbon for use in the present invention is 1,1,1,2-tetrafluoroethane (R-134a).
It is possible to carry out biomass extraction using other solvents such as chlorofluorocarbons (xe2x80x9cCFC""sxe2x80x9d) and hydrochlorofluorocarbons (xe2x80x9cHCFC""sxe2x80x9d), and/or mixtures of solvents. CFC""s and HCFC""s are not approved for food use and consequently are rarely employed in extraction processes in which the depleted biomass residue is intended as e.g. an animal feed.
Known extraction processes using solvents are normally carried out in closed-loop extraction equipment. A typical example 10 of such a system is shown schematically in FIG. 1.
In this typical system, liquefied solvent is allowed to percolate by gravity in downflow through a bed of biomass held in vessel 11. Thence it flows to evaporator 12 where the volatile solvent is vaporised by heat exchange with a hot fluid. The vapour from evaporator 12 is then compressed by compressor 13.
The compressed vapour is next fed to a condenser 14 where it is liquefied by heat exchange with a cold fluid. The liquefied solvent is then optionally collected in intermediate storage vessel 15 or returned (line 16) directly to the extraction vessel 1 to complete the circuit.
One of the key areas of concern relating to the use of solvents such as are used in biomass extraction processes is the level of residual solvent on the biomass material after extraction is complete. High levels of residual HFC (or other) solvent may be regarded as undesirable from a number of aspects:
loss of HFC to atmosphere
loss of HFC from the recycle process potentially increasing top-up costs
landfill, incineration, composting and other biomass disposal regulatory
issues
suitability of depleted biomass for use as an animal feedstuff supplement
In order to improve the rate of solvent extraction, the biomass is usually chopped or ground in some manner in order to increase the surface area in contact with the extraction solvent. Whilst beneficially increasing the rate of extraction of the desired components during biomass extraction, this increased surface area acts to increase the quantity of solvent that can remain adsorbed onto and in the biomass after extraction. Clearly some cost-effective method of achieving acceptable residual HFC solvent levels in the exhausted biomass would be of significant value in the development of the technology.
A combination of evacuation and heat (e.g. using a heating jacket surrounding extraction vessel 11) may act to reduce the levels of residual solvent in the biomass over a period of time. This method however has a number of potential disadvantages including:
prolonged evacuation time to achieve low residue levels
poor heat transfer into the packed-bed of biomass from the vessel jacket resulting in uneven heating and possible thermal deterioration of the biomass (charring, caramelisation etc.)
Such charring and caramelisation are particularly undesirable since they may adversely impact the commercial value of the biomass extracts.
According to the invention in a first aspect there is provided apparatus for removing solvent residue from a bed of biomass comprising an extraction vessel for containing biomass that permits a solvent or a solvent mixture to contact biomass therein to effect extraction; a source of steam selectively connectable to supply steam to the biomass in the extraction vessel; a separator for separating the steam that has contacted the biomass in the extraction vessel and solvent entrained therewith; and a delivery line for steam/solvent selectively interconnectable between the extraction vessel and the separator to permit passage of steam and solvent entrained therewith to the separator.
In use of the apparatus steam advantageously strips solvent (that typically is an HFC such as 1,1,1,2-tetrafluoroethane, or mixtures of HFC""s) from the biomass. The solvent is then entrained in the steam and conveyed to the separator, where the steam and the solvent are separated to allow recovery and/or disposal of the solvent.
In use of the apparatus the steam may contact the biomass once; or the apparatus may optionally include means such as pipework and valves permitting the steam to contact the biomass more than once.
Preferably the source of steam supplies steam at or near atmospheric pressure.
Alternatively the source of steam supplies steam at super-atmospheric pressure.
In preferred embodiments the separator is or includes an adsorbent material for removing solvent entrained with the steam. More preferably the adsorbent material is or includes activated carbon. These features conveniently make the solvent available for recycling, or permit ready disposal of the solvent.
The adsorbent filter material may also act to reduce any volatilised organic materials present in the depleted biomass, again improving the quality of the effluent condensate. Depending on the nature of the adsorbent and on economic attractiveness, a concentrated stream of recovered HFC may be obtained from thermal regeneration of the adsorbent. Alternatively, the loaded adsorbent provides a convenient, compact and cost-effective package for appropriate disposal.
Optional forms of apparatus of the invention, include a condenser, either upstream or downstream of the separator, to condense the stream and facilitate separation of the steam and solvent from one another.
A preferred form of extraction vessel includes a cylindrical chamber closed at either end and having an inlet at one end and an outlet at its other end, the hollow interior of the chamber being for containing biomass, the inlet being selectively connectable to a source of solvent and a source of steam; and the outlet being selectively connectable as part of a circuit for recovering biomass extract to a vacuum or to the separator. The purpose of the option connection to a vacuum or to suction is to permit evacuation of the vessel at the end of a biomass extraction, thereby removing from the vessel the bulk of the solvent therein. This means that the steam serves primarily to strip solvent that is adsorbed onto the surface of the biomass.
The vacuum connection may also advantageously be used to purge the vessel of e.g. water, when it is required to extract a new bed of biomass.
Preferably the extraction vessel is in use vertical, with the inlet at its lower end and the outlet at its upper end. This advantageously allows the charging of the extraction vessel with a packed bed of biomass. Such a packed bed may advantageously occupy substantially the entire cross-section of part of the vessel.
This arrangement has advantages in reducing the energy consumption of biomass extraction apparatus as shown in FIG. 1.
The extraction vessel may optionally include a selectively operable condenser (cooling) jacket. This allows steam condensing advantageously to occur within the extraction vessel.
Preferably the extraction vessel includes thermal insulation. This advantageously reduces heat transfer from the vessel during both the biomass extraction and desorption (solvent recovery) processes.
According to a second aspect of the invention there is provided a method of removing solvent residues from a bed of biomass, comprising contacting the biomass with steam; passing the steam and solvent entrained therewith to a separator; and separating the steam and the solvent from one another in the separator. This method may conveniently be practised using apparatus as defined herein.
Further, advantageous features of the method are that the step of separating includes contacting an adsorbent with the steam/solvent mixture; the step of condensing the steam before or after passing it to the separator; the step of condensing the steam before passing it to the separator; and wherein the condensing occurs within a vessel containing the biomass; the step of contacting the biomass with steam occurs in a vessel, and the method includes the step of partially or substantially evacuating the vessel before the steam contacts the biomass; the step of separating includes contacting an adsorbent with the steam/solvent mixture; and including the step of heating the adsorbent to recover solvent therefrom; the step of separating includes contacting an adsorbent with the steam/solvent mixture; and including the step of disposing of the adsorbent and the solvent therewith; and/or the step of condensing the steam includes recovering heat from the condensate and using the recovered heat to pre-heat water for steam generation.
The method of the invention is superior to simple heating and/or evacuation, because
the temperature of the biomass is raised rapidly and directly through intimate contact with steam
steam flow acts to maintain a low partial pressure of HFC solvent in the vapour space around the biomass thus aiding transfer of HFC from the biomass to the steam stream
water from the steam is likely to act to displace HFC from the surface of the biomass through preferential adsorption thus further improving the rate and effectiveness of desorption relative to a flow of other gases such as nitrogen.